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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>2.</b></font><b><font face="Arial" size="2" color="#FFFFFF">3</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">WAN Design</font></strong></td>
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<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
2.3.4</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">Three-layer design model</font></strong></td>
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A systematic approach is needed when many locations
must be joined. A hierarchical solution with three layers offers many
advantages.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12">
<p>Imagine an enterprise that is operational in every country of the
European Union and has a branch in every town with a population over
10,000. Each branch has a LAN, and it has been decided to interconnect
the branches. A mesh network is clearly not feasible because nearly
500,000 links would be needed for the 900 centers. A simple
star will be very difficult to implement because it needs a router
with 900 interfaces at the hub or a single interface that carries 900
virtual circuits to a packet-switched network.</p>
<p>Instead, consider a hierarchical design model. A group of LANs in an
area are interconnected, several areas are interconnected to form a
region, and the various regions are interconnected to form the core of
the WAN. </p>
<p>The area could be based on the number of locations to be connected
with an upper limit of between 30 and 50. The area would have a star
topology,
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12">
with the hubs of the stars linked to form the region.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12">
<img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12">
Regions could be geographic, connecting between three and ten areas,
and the hub of each region could be linked point-to-point.
<img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12"></p>
<p>This three-layer model follows the hierarchical design used in telephone
systems. The links connecting the various sites in an area that
provide access to the enterprise network are called the access links
or access layer of the WAN. Traffic between areas is distributed by
the distribution links, and is moved onto the core links for transfer
to other regions, when necessary.</p>
<p>This hierarchy is often useful when the network traffic mirrors the
enterprise branch structure and is divided into regions, areas, and
branches. It is also useful when there is a central service to which
all branches must have access, but traffic levels are insufficient to
justify direct connection of a branch to the service.</p>
<p>The LAN at the center of the area may have servers providing
area-based as well as local service. Depending on the traffic volumes
and types, the access connections may be dial up, leased, or Frame
Relay. Frame Relay facilitates some meshing for redundancy without
requiring additional physical connections. Distribution links could be
Frame Relay or ATM, and the network core could be ATM or leased line.</p>
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